1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus including a transfer device such as a copying machine, a laser printer, a facsimile machine, or other similar image forming apparatus, and more particularly to a transfer device in which a toner image formed on a photoreceptor is primarily transferred to an intermediate transfer element and is secondarily transferred from the intermediate transfer element to a recording medium such as a sheet, etc.
2. Discussion of the Background
An image forming apparatus including a transfer device, in which a toner image formed on a photoreceptor is primarily transferred to an intermediate transfer element and is secondarily transferred from the intermediate transfer element to a recording medium, has been widely used. For example, Japanese Laid-open Patent Publication Nos. 10-186879 and 11-161061 describe such background image forming apparatuses.
In a background multi-color image forming apparatus including a transfer device, latent images formed on a photoreceptor are developed with toner of different colors by color developing devices and are formed into toner images of different colors. The toner images of different colors are sequentially transferred from the photoreceptor to a transfer belt as an intermediate transfer element by a primary transfer device while being superimposed upon each other on the transfer belt in a primary transfer process.
Subsequently, the superimposed color toner image on the transfer belt is moved to a secondary transfer device. Until all the toner images of different colors are primarily transferred to the transfer belt, the color toner image already transferred to the transfer belt just passes the secondary transfer device. Upon completion of the primary transfer process, a secondary transfer process is started by the secondary transfer device.
FIG. 7A illustrates one type of a background secondary transfer device that performs a secondary transfer process. The secondary transfer device of FIG. 7A includes a transfer roller 50a and a back-up roller 60a. A voltage is applied to a core metal of the transfer roller 50a from a high-voltage power supply 40a. The back-up roller 60a is provided opposite to the transfer roller 50a via a transfer belt 100a and is electrically grounded. A superimposed color toner image on the transfer belt 100a is transferred to a recording medium “S,” which is fed to a transfer nip part formed between the transfer belt 100a and the transfer roller 50a in synchronism with the movement of the superimposed color toner image, under the influence of a transfer electric field formed by the transfer roller 50a. 
FIGS. 7B and 7C illustrate other types of background secondary transfer devices. The secondary transfer device of FIG. 7B includes a transfer roller 50b, a back-up roller 60b, and a contact roller 70. The back-up roller 60b is provided opposite to the transfer roller 50b via a transfer belt 100b. The contact roller 70 is rotatably provided in contact with an upper circumferential surface of the back-up roller 60b. A voltage is applied to the contact roller 70 from a power supply 40b. 
The secondary transfer device of FIG. 7C includes a transfer roller 50c and a back-up roller 60c. The back-up roller 60c is provided opposite to the transfer roller 50c via a transfer belt 100c. A voltage is applied to a core metal of the back-up roller 60c from a power supply 40c. 
In the secondary transfer device of FIG. 7B, a superimposed color toner image on the transfer belt 100b is transferred to a recording medium “S,” which is fed to a transfer nip part formed between the transfer belt 100b and the transfer roller 50b, under the influence of a transfer electric field formed by the contact roller 70. In the secondary transfer device of FIG. 7C, a superimposed color toner image on the transfer belt 100c is transferred to a recording medium “S,” which is fed to a transfer nip part formed between the transfer belt 100c and the transfer roller 50c, under the influence of a transfer electric field formed by the back-up roller 60c. 
In the secondary transfer device of FIG. 7A in which a transfer electric field is formed by the transfer roller 50a, when an electric resistance of the recording medium “S” is low, electric current may flow into the recording medium “S” and leak to a member other than the transfer belt 100a which contacts the recording medium “S,” resulting in a reactive electric current. In this condition, because an amount of electric current used for forming the transfer electric field decreases, the transfer electric field is reduced. Thus, an image transfer efficiency tends to be decreased.
In the secondary transfer devices of FIGS. 7B and 7C in which a transfer electric field is formed on the side of the back-up roller, a problem resulting from the decrease of image transfer efficiency may be obviated. However, when forming a transfer electric field by the contact roller 70, the resistance of a semiconducting tube provided in a surface portion of the back-up roller 60b tends to be uneven. As a result, the transfer electric field tends to be relatively unstable. Further, when forming a transfer electric field by the core metal of the back-up roller 60c, a problem resulting from the resistance unevenness does not occur. However, when the width of the recording medium “S” is small, an excessive amount of electric current flows to an area of the transfer belt 100c outside the recording medium “S” where the transfer roller 50c is in direct contact with the transfer belt 100c. As a result, damage may be caused to the device, and a desired transfer electric field may not be formed.